Amorphous materials are a new class of engineering material, which have a unique combination of high strength, elasticity, corrosion resistance and processability from the molten state. Amorphous materials differ from conventional crystalline alloys in that their atomic structure lacks the typical long-range ordered patterns of the atomic structure of conventional crystalline alloys. Amorphous materials are generally processed and formed by cooling a molten alloy from above the melting temperature of the crystalline phase (or the thermodynamic melting temperature) to below the “glass transition temperature” of the amorphous phase at “sufficiently fast” cooling rates, such that the nucleation and growth of alloy crystals is avoided. As such, the processing methods for amorphous alloys have always been concerned with quantifying the “sufficiently fast cooling rate”, which is also referred to as “critical cooling rate”, to ensure formation of the amorphous phase.
The “critical cooling rates” for early amorphous materials were extremely high, on the order of 106° C./sec. As such, conventional casting processes were not suitable for such high cooling rates, and special casting processes such as melt spinning and planar flow casting were developed. Due to the crystallization kinetics of those early alloys being substantially fast, extremely short time (on the order of 10−3 seconds or less) for heat extraction from the molten alloy were required to bypass crystallization, and thus early amorphous alloys were also limited in size in at least one dimension. For example, only very thin foils and ribbons (order of 25 microns in thickness) were successfully produced using these conventional techniques. Because the critical cooling rate requirements for these amorphous alloys severely limited the size of parts made from amorphous alloys, the use of early amorphous alloys as bulk objects and articles was limited.
Over the years it was determined that the “critical cooling rate” depends strongly on the chemical composition of amorphous alloys. Accordingly, a great deal of research was focused on developing new alloy compositions with much lower critical cooling rates. Examples of these alloys are given in U.S. Pat. Nos. 5,288,344; 5,368,659; 5,618,359; and 5,735,975, each of which is incorporated herein by reference. These amorphous alloy systems, also called bulk-metallic glasses or BMGs, are characterized by critical cooling rates as low as a few ° C./second, which allows the processing and forming of much larger bulk amorphous phase objects than were previously achievable.
With the availability of low “critical cooling rate” BMGs, it has become possible to apply conventional casting processes to form bulk articles having an amorphous phase. Over the past several years, a number of companies, including LiquidMetal Technologies, Inc. have undertaken an effort to develop commercial manufacturing technologies for the production of net shape metallic parts fabricated from BMGs. For example, manufacturing methods such as permanent mold metal die-casting and injection casting into heated molds are currently being used to fabricate commercial hardware and components such as electronic casings for standard consumer electronic devices (e.g., cell phones and handheld wireless devices), hinges, fasteners, medical instruments and other high value added products. However, even though bulk-solidifying amorphous alloys provide some remedy to the fundamental deficiencies of solidification casting, and particularly to the die-casting and permanent mold casting processes, as discussed above, there are still issues which need to be addressed. First and foremost, there is a need to make these bulk objects from a broader range of alloy compositions. For example, presently available BMGs with large critical casting dimensions capable of making large bulk amorphous objects are limited to a few groups of alloy compositions based on a very narrow selection of metals, including Zr-based alloys with additions of Ti, Ni, Cu, Al and Be and Pd-based alloys with additions of Ni, Cu, and P, which are not necessarily optimized from either an engineering or cost perspective.
In addition, the current processing technology requires a great deal of expensive machinery to ensure appropriate processing conditions are created. For example, most shaping processes require a high vacuum or controlled inert gas environment, induction melting of material in a crucible, pouring of metal to a shot sleeve, and pneumatic injection through a shot sleeve into gating and cavities of a rather elaborate mold assembly. These modified die-casting machines can cost several hundreds of thousands of dollars per machine. Moreover, because heating a BMG has to date been accomplished via these traditional, slow thermal processes, the prior art of processing and forming bulk-solidifying amorphous alloys has always been focused on cooling the molten alloy from above the thermodynamic melting temperature to below the glass transition temperature. This cooling has either been realized using a single-step monotonous cooling operation or a multi-step process. For example, metallic molds (made of copper, steel, tungsten, molybdenum, composites thereof, or other high conductivity materials) at ambient temperatures are utilized to facilitate and expedite heat extraction from the molten alloy. Because the “critical casting dimension” is correlated to the critical cooling rate, these conventional processes are not suitable for forming larger bulk objects and articles of a broader range of bulk-solidifying amorphous alloys. In addition, it is often necessary to inject the molten alloy into the dies at high-speed, and under high-pressure, to ensure sufficient alloy material is introduced into the die prior to the solidification of the alloy, particularly in the manufacture of complex and high-precision parts. Because the metal is fed into the die under high pressure and at high velocities, such as in high-pressure die-casting operation, the flow of the molten metal becomes prone to Rayleigh-Taylor instability. This flow instability is characterized by a high Weber number, and is associated with the break-up of the flow front causing the formation of protruded seams and cells, which appear as cosmetic and structural micro-defects in cast parts. Also, there is a tendency to form a shrinkage cavity or porosity along the centerline of the die-casting mold when unvitrified liquid is trapped inside a solid shell of vitrified metal.
Attempts to remedy the problems associated with rapidly cooling the material from above the equilibrium melting point to below the glass transition were mostly focused on utilizing the kinetic stability and viscous flow characteristics of the supercooled liquid. Methods have been proposed that involve heating glassy feedstock above the glass transition where the glass relaxes to a viscous supercooled liquid, applying pressure to form the supercooled liquid, and subsequently cooling to below glass transition prior to crystallizing. These attractive methods are essentially very similar to those used to process plastics. In contrast to plastics however, which remain stable against crystallization above the softening transition for extremely long periods of time, metallic supercooled liquids crystallize rather rapidly once relaxed at the glass transition. Consequently, the temperature range over which metallic glasses are stable against crystallization when heated at conventional heating rates (20° C./min) are rather small (50-100° C. above glass transition), and the liquid viscosity within that range is rather high (109-107 Pa s). Owing to these high viscosities, the pressures required to form these liquids into desirable shapes are enormous, and for many metallic glass alloys could exceed the pressures attainable by conventional high strength tooling (<1 GPa). Metallic glass alloys have recently been developed that are stable against crystallization when heated at conventional heating rates up to considerably high temperatures (165° C. above glass transition). Examples of these alloys are given in U.S. Pat. Appl. 20080135138 and articles to G. Duan et al. (Advanced Materials, 19 (2007) 4272) and A. Wiest (Act Materialia, 56 (2008) 2525-2630), each of which is incorporated herein by reference. Owing to their high stability against crystallization, process viscosities as low as 105 Pa-s become accessible, which suggests that these alloys are more suitable for processing in the supercooled liquid state than traditional metallic glasses. These viscosities however are still substantially higher than the processing viscosities of plastics, which typically range between 10 and 1000 Pa-s. In order to attain such low viscosities, the metallic glass alloy should either exhibit an even higher stability against crystallization when heated by conventional heating, or be heated at an unconventionally high heating rate which would extend the temperature range of stability and lower the process viscosity to values typical of those used in processing thermoplastics.
A few attempts have been made to create a method of instantaneously heating a BMG up to a temperature sufficient for shaping, thereby avoiding many of the problems discussed above and simultaneously expanding the types of amorphous materials that can be shaped. For example, U.S. Pat. Nos. 4,115,682 and 5,005,456 and articles to A. R. Yavari (Materials Research Society Symposium Proceedings, 644 (2001) L12-20-1, Materials Science & Engineering A, 375-377 (2004) 227-234; and Applied Physics Letters, 81(9) (2002) 1606-1608), the disclosures of each of which are incorporated herein by reference, all take advantage of the unique conductive properties of amorphous materials to instantaneously heat the materials to a shaping temperature using Joule heating. However, thus far these techniques have focused on localized heating of BMG samples to allow for only localized forming, such as the joining (i.e., spot welding) of such pieces, or the formation of surface features. None of these prior art methods teach how to uniformly heat the entire BMG specimen volume in order to be able to perform global forming. Instead, all those prior art methods anticipate temperature gradients during heating, and discuss how these gradients could affect local forming. For instance, Yavari et al. (Materials Research Society Symposium Proceedings, 644 (2001) L12-20-1) write: The external surfaces of the BMG specimen being shaped, whether in contact with the electrodes or with the ambient (inert) gas in the shaping chamber, will be slightly cooler than the inside as the heat generated by the current dissipates out of the sample by conduction, convection or radiation. On the other hand, the outer surfaces of samples heated by conduction, convection or radiation are slightly hotter than the inside. This is an important advantage for the present method as crystallization and or oxidation of metallic glasses often begin first on outer surfaces and interfaces and if they are slightly below the temperature of the bulk, such undesirable surface crystal formation may be more easily avoided.”
Another drawback of the limited stability of BMGs against crystallization above the glass transition is the inability to measure thermodynamic and transport properties, such as heat capacity and viscosity, over the entire range of temperatures of the metastable supercooled liquid. Typical measurement instruments such as Differential Scanning calorimeters, Thermo-Mechanical Analyzers, and Coquette Viscometers rely on conventional heating instrumentation, such as electric and induction heaters, and are thus capable of attaining sample heating rates that are considered conventional (typically <100° C./min). As discuss above, metallic supercooled liquids can be stable against crystallization over a limited temperature range when heated at a conventional heating rate, and thus the measureable thermodynamic and transport properties are limited to within the accessible temperature range. Consequently, unlike polymer and organic liquids which are very stable against crystallization and their thermodynamic and transport properties are measureable throughout the entire range of metastability, the properties of metallic supercooled liquids are only measureable to within narrow temperature ranges just above the glass transition and just below the melting point.
Recently, a method has been developed which overcomes many of the limitations of these conventional methods by uniformly heating, rheologically softening, and thermoplastically forming metallic glasses rapidly into a net shape using a rapid capacitor discharge forming (RCDF) tool. (See, e.g., U.S. Pat. Pub. No. US-2009-0236017-A1, the disclosure of which is incorporated herein by reference.) The RCDF method utilizes the discharge of electrical energy stored in a capacitor to uniformly and rapidly heat a sample or charge of metallic glass alloy to a predetermined “process temperature” between the glass transition temperature of the amorphous material and the equilibrium melting point of the alloy in a time scale of several milliseconds or less. However, in this method the application of force to shape the heated sample into high quality amorphous bulk articles is done through conventional techniques, which are not optimal.
Accordingly, a need exists to find a novel approach to shape the heated BMG specimen that exploits the electric field generated in the instantaneous RCDF heating method.